Seismic surveying is used to map the geology of formations in a survey area by way of detecting the elastic properties of different geological features. Marine seismic exploration investigates and maps the structure and character of subsurface geological formations underlying a body of water. Marine seismic surveying typically uses a marine survey vessel to tow a seismic source through a body of water over the survey area whilst actuating the seismic source at selected intervals of time. Making a marine seismic survey of the structure and character of a subsea geological formation relies on intermittently activating the seismic source to generate seismic waves which travel through the body of water and the seabed before being refracted through the earth or reflected at interfaces associated with geological features within the subsea geological formation. The various seismic signals are received by a plurality of seismic signal receivers or sensors and recorded for further analysis. In a marine environment, one or more streamer cables may be towed by the survey vessel or by another vessel over the survey area, each streamer cable comprising a plurality of seismic signal receivers arranged at spaced apart locations along the length of the streamer cable. Alternatively or additionally, the seismic signal receivers may be arranged on the sea bed and the seismic source may be towed behind a separate vessel
There are two types of seismic sources in use at this time. The first type is an ‘impulsive’ seismic source which is ‘shot’ at intermittent time intervals to produce ‘pressure waves’ or ‘P-waves’. Examples of an impulsive seismic source are explosive devices such as ‘air guns’, ‘gas guns’ or ‘sleeve exploders’ or implosive devices. When an airgun is shot, it discharges air under very high pressure into the water. The discharged air forms a bubble which oscillates at a given frequency which is a function of the size of the airgun and how far below the surface of the water the airgun is located at the time that it is fired. This air bubble generates a pressure wave that expands outwards into a large volume until it interacts with a geological feature and generates a reflected seismic signal. One problem with the use of air guns is the interaction of the pulse of air with the surface of the water which creates a reflected ‘ghost signal’ that interferes with the reflected seismic signals from the geological features of interest that are being surveyed. The seismic signal generated by impulsive seismic sources has a waveform that cannot be precisely controlled and is either on or off.
Due to the physical limitations associated with generating and collapsing a bubble of air in seawater, another problem with the use of conventional impulsive seismic sources is that airguns only generate significant energy above around 5 Hz. One of the limiting factors for air guns (and water guns) with respect to a lowest practical output frequency is the size of the chamber used to store the pressurized gas, air or water. The low frequency energy is further reduced by the destructive interference of the ghost signal reflected from the surface of the water. Consequently, conventional airguns are only capable of generating a seismic signal having bandwidth in the range of 10 to 250 Hz. Attempts have been made to overcome this inherent problem of not being able to generate low frequency seismic signals using airguns by making them bigger or towing them at lower depths below the surface of the water. However, the pressure on the air bubbles generated when conventional airguns are towed more deeply under the water is increased making it more difficult to generate a large air bubble of lower frequency, with the result that the capacity of the air guns must also be increased to generate a signal of equivalent size. Attempts have also been made to operate multiple air guns of different sizes in an attempt to cancel out unwanted ghost signals. Despite these attempts, a particular limitation to impulsive seismic sources known in the art is that they are not capable of generating low frequency seismic energy, typically less than about 5 Hz.
The second type of seismic source is a marine vibrator that generates a waveform that can be controlled (see, for example, U.S. Pat. No. 3,863,202, U.S. Pat. No. 3,349,367, U.S. Pat. No. 4,635,747, U.S. Pat. No. 4,918,668, and U.S. Pat. No. 8,102,731). Marine vibrators typically include a pneumatic or hydraulic actuator which causes an acoustic piston to vibrate at a selected frequency. The vibrations of the acoustic vibrator produce pressure differentials in the water which generate seismic pulses free from spurious bubbles. Vibrator-type seismic energy sources are programmed through control signals to generate energy in the seismic frequency bandwidth of between 10 and 190 Hz. At these frequencies, the reactive mass or diaphragm must be relatively large, and the amount of motion that must be imparted to the radiating surface is also relatively large. Controlling such motion so that it faithfully corresponds to the electrical control signal has proven to be difficult. Thus, one of the limiting factors of a seismic energy source such as a marine vibrator is the power required to move the driving element at low frequencies and to maintain a reasonably pure tone, because hydraulically driven devices in the vibrator may have difficulty shifting the required fluid volume for such low frequencies. Consequently, a particular limitation to vibrator seismic sources known in the art is that they are not capable of generating low frequency seismic energy, typically less than about 10 Hz.
Whilst it is relatively easy to detect a geological interface where there are very sudden changes in the geological properties of the earth using conventional mid to high frequency seismic sources, it is only possible to detect gradual changes between the various layers in a formation at low frequencies. This limited range of bandwidths available with conventional seismic sources has the result that information regarding gradual changes in the geological features cannot be captured. When seismic survey data is collected, it is subjected to processing to attempt to recreate all the physical properties of the earth that relate to the propagation of seismic waves. One of the modelling techniques used during processing is called ‘Full Waveform Inversion’. Because it is not possible to measure low frequency information below 5 Hz using conventional seismic sources, the seismic data that is acquired using conventional seismic sources is subjected to months of processing to estimate such parameters as the velocity and density to allow Full Waveform Inversion to commence. In the absence of low-frequency observed data below 5 Hz, full wave form inversion algorithms will not converge on the correct earth model due to the problem of local minima. Current approaches use conventional velocity tomography to create a ‘starting model’ for Full Waveform Inversion containing low-frequency information not measured in the survey. This approach has several major shortcomings. It requires a full seismic processing and imaging workflow to be applied before the tomography step can be conducted. This is time consuming and expensive. The result is a model with features in the range of 0 to 2 Hz. However, tomography still cannot resolve features in the range of 2 to 5 Hz which means that the result is a poor starting model for Full Waveform Inversion and the local minima problem is not satisfactorily addressed.
There remains a need in the art for an alternative marine seismic data source capable of supplying the lower frequencies required to more accurately characterise subsea geological features.